Microelectronic devices are used in cell phones, pagers, personal digital assistants, computers, and many other products. A packaged microelectronic device can include a microelectronic die, an interposer substrate or lead frame attached to the die, and a molded casing around the die. The microelectronic die generally has an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The bond-pads are coupled to terminals on the interposer substrate or lead frame. The interposer substrate can also include ball-pads coupled to the terminals by traces in a dielectric material. An array of solder balls is configured so that each solder ball contacts a corresponding ball-pad to define a “ball-grid” array. Packaged microelectronic devices with ball-grid arrays are generally higher grade packages that have lower profiles and higher pin counts than conventional chip packages that use a lead frame.
Packaged microelectronic devices are typically made by (a) forming a plurality of dies on a semiconductor wafer, (b) cutting the wafer to singulate the dies, (c) attaching individual dies to an interposer substrate, (d) wire-bonding the bond-pads to the terminals of the interposer substrate, and (e) encapsulating the dies with a molding compound. It is time consuming and expensive to mount individual dies to individual interposer substrates. Also, as the demand for higher pin counts and smaller packages increases, it become more difficult to (a) form robust wire-bonds that can withstand the forces involved in molding processes and (b) accurately form other components of die level packaged devices. Therefore, packaging processes have become a significant factor in producing semiconductor and other microelectronic devices.
Another process for packaging microelectronic devices is wafer-level packaging. In wafer-level packaging, a plurality of microelectronic dies are formed on a wafer and then a redistribution layer is formed on top of the dies. The redistribution layer has a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and traces coupled to individual ball-pads of the ball-pad arrays. Each ball-pad array is arranged over a corresponding microelectronic die, and the ball-pads in each array are coupled to corresponding bond-pads on the die by the traces in the redistribution layer. After forming the redistribution layer on the wafer, a stenciling machine deposits discrete blocks of solder paste onto the ball-pads of the redistribution layer. The solder paste is then reflowed to form solder balls or solder bumps on the ball-pads. After formation of the solder balls on the ball-pads, the wafer can be cut to singulate the dies. Microelectronic devices packaged at the wafer-level can have high pin counts in a small area, but they are not as robust as devices packaged at the die-level.
Packaged microelectronic devices can also be produced by “build-up” packaging. For example, a sacrificial substrate can be attached to a panel including a plurality of microelectronic dies and an organic filler that couples the dies together. The sacrificial substrate is generally a ceramic disc, and it is attached to the active side of the microelectronic dies. Next, the back side of the microelectronic dies is thinned, and then a ceramic layer is attached to the back side. The sacrificial substrate is then removed from the active side of the dies and build-up layers or a redistribution layer can be formed on the active side of the dies. Packaged devices using a build-up approach on a sacrificial substrate provide high pin counts in a small area and a reasonably robust structure.
The build-up packaging process, however, has several drawbacks. For example, the process is relatively expensive and may not be used on equipment set up for circular substrates. Furthermore, the resulting packaged microelectronic devices do not have an effective mechanism for dissipating heat, which can significantly impair the electrical performance of the device. Accordingly, there is a need for an efficient and cost-effective process to package microelectronic dies that have heat dissipation mechanisms.